The invention relates to the field of micro imaging and, in particular, to methods of recalibration to compensate for thermal drift between a micro-imaging system and a sample under investigation.
Semiconductor integrated circuits (ICs) are reverse-engineered for the purposes of validation and product quality assurance. Typically a large group of high magnification tile images representative of a sample IC are acquired using a high magnification micro-imaging system in between deconstructive steps. Subsequent processing of the tile images includes their assembly into one or more photo-mosaics. Each photo-mosaic is representative of the IC sample, or portion thereof, at a particular deconstructive step.
Representative dimensions that have to be resolved by the micro-imaging system are related to the width of traces, which are connections between components on the ICs. Trace width reduction is a goal sought in the semiconductor manufacturing industry to provide increased integration, increased switching speed, reduction in drive voltages, etc. Today typical trace widths are in the micron and submicron range.
The time required for micro imaging an IC sample at each deconstructive step is typically on the order of hours or days. It is well known in the art of material sciences that all known materials are subject to temperature induced deformation. The degree of deformation is dependent on a particular material""s coefficient of thermal expansion. A high magnification optical microscope is commonly used in micro imaging. The microscope typically has an arm supporting high magnification optic elements above the sample IC. It has been observed that the components of the micro-imaging system are subject to temperature induced deformation. It has also been observed that the temperature-induced deformation is time dependent, typically having a linear variation. This phenomenon is commonly referred to as xe2x80x9cthermal driftxe2x80x9d. Thermal drift between the optics and the sample IC during micro imaging can cause misalignment between images acquired at different times. It will be understood by those skilled in the art that an imaging system other than optical, such as a beam instrument as used in a Scanning Electron Microscope (SEM) or Focus Ion Beam (FIB), could also be used.
Considering the size of the arm of the optical microscope and coefficient of thermal expansion of the materials used in manufacturing the arm (such as aluminum), thermal drift can sometimes cause a misalignment in excess of 20 microns cumulative between images. Therefore, thermal drift can be a significant impediment to acquiring high magnification images and assembling the images into photo-mosaics representative of a surface of interest of the IC sample.
Misalignments between the optical system and the IC sample during micro-imaging result in either an excess of overlap between the acquired images, or the formation of inter-image gaps. Having excess overlap leads to waste processing time. Having incomplete mosaics due to inter-image gaps results in an inability to validate the IC design. One option for reducing thermal drift is to wait for the temperature of the sample IC and imaging apparatus to stabilize, assuming stable temperature conditions. However, suitable conditions are very rare. Another option is to enforce dynamic temperature control during the micro-imaging process. This is an expensive option that is difficult to achieve in practice, at least partly because the image acquisition process itself generates heat because the sample IC is positioned for each image acquisition by an electromechanical drive mechanism that generates heat when operated. Other factors related to temperature control are heat capacity and heat conductivity coefficients of materials, which impose limits on how quickly, and at what cost dynamic temperature control can be effected.
There therefore exists a need for methods and apparatus for dynamically recalibrating a micro-imaging system to compensate for thermal drift.
It is an object of the invention to provide a method of determining thermal drift between a micro-imaging system and a sample to compensate for temperature-induced deformation.
It is another object of the invention to ensure compensation for misalignment between a micro-imaging system and an imaged sample based on measured thermal drifts determined during a photo-mosaic acquisition period.
It is a further object of the invention to provide a method of determining thermal drift between the micro-imaging system and the sample at predetermined time intervals during a photo-mosaic acquisition period.
It is a further object of the invention to provide a method of determining thermal drift between the micro-imaging system and the sample based on an adaptive recalibration time interval responsive to a rate of thermal drift.
In accordance to one aspect of the invention a method is provided for determining a measure of thermal drift between the micro-imaging system and a sample. The method includes an initial calibration step performed prior to an acquisition of a series of high magnification images of a surface of interest of the sample. The initial calibration includes positioning a pre-selected calibration location on the sample in a field-of-view of the micro-imaging system, determining a reference calibration focus setting by focusing the micro-imaging system on the sample at the pre-selected calibration location and capturing a reference calibration image. The recalibration is triggered during the acquisition of the images to determine the thermal drift. The thermal drift determination includes repositioning the pre-selected calibration location in the field-of-view of the micro-imaging system, determining a recalibration focus setting, capturing a recalibration image, determining a planar shift from a correlation between the reference calibration image and the recalibration image. The determined planar shift and a difference between the reference calibration focus setting and the recalibration focus setting represent the measure of thermal drift between the micro-imaging system and the sample.
A method is provided for acquiring high magnification tile images of an integrated circuit sample using an optical system subject to thermal drifts. A field of view of a high magnification power optical system is positioned over a surface of the sample at a predetermined location. A tile image of the surface is captured and stored. On detecting a trigger event, a thermal drift between the optical system and the sample is determined with respect to a predetermined calibration location on the surface of the sample. The acquisition of tile images is continued in accordance with predefined rules respecting a degree of thermal drift. The rules include aborting tile image acquisition in the event of excessive thermal drift, recapturing all tile images since a last recalibration in the case of a large thermal drift, and otherwise continuing tile image acquisition.
In accordance with another aspect of the invention the trigger event includes the expiration of a recalibration time interval and the recalibration time interval is adaptively varied based on rules respecting the degree of the thermal drift. As such the recalibration time interval is increased when thermal drift is slight and decreased when thermal drift is significant. Other rules ensure that recalibrations are performed during the tile image acquisition period, and that recalibrations occupy only a certain amount of processing time.
The invention also provides a micro-imaging system for acquiring high magnification tile images of a sample while the system is subject to thermal drift, the tile images being used to construct a seamless photo-mosaic of a surface of interest of the sample. The micro-imaging system comprises means for positioning the surface of interest in a field of view of a micro-imaging system at a location on the sample; means for storing a position of the location; means for focusing the field of view of the micro-imaging system on the surface of interest; means for capturing an image of the surface of interest; means for storing the captured image; means for detecting a trigger event for determining a thermal drift between the micro-imaging-system and the sample; means for determining a thermal drift between the micro-imaging system and the sample in response to the triggering event; and means for controlling the micro-imaging system in accordance with predefined rules respecting the capture of the tile images, the rules being related to an extent of the thermal drift since a last trigger event.
The means for positioning the surface of interest in a field of view of a micro-imaging system at a location on the sample comprises a stage of the micro-imaging system. Algorithms that are executed by a computer workstation control the stage.
The means for determining the thermal drift comprises an algorithm for capturing and storing an in-focus calibration image of a predetermined calibration location on the surface of interest and a focus setting used to capture the calibration image. An algorithm that controls the micro-imaging system returns the system to the calibration location on detection of the trigger event and captures and stores an in-focus recalibration image of the predetermined calibration location on the surface of interest and a focus setting used to capture the in-focus recalibration image.
The means for determining the thermal drift further comprises an algorithm for cross-correlating the calibration image and the recalibration image to determine a planar shift of the micro-imaging system with respect to the surface of interest, and for computing a difference between the calibration and the recalibration focus settings. The algorithm for cross-correlating the calibration image uses a Fourier transform of the calibration image and a Fourier transform of the recalibration image to determine a planar shift along the surface of interest of the recalibration image with respect to the calibration image.
The means for interpreting the predefined rules comprises an algorithm for comparing the extent of the thermal drift to at least one threshold to determine a next action dependent on the extent of the thermal drift with respect to the at least one threshold.
A re-settable clock counter is preferably used for tracking the recalibration interval. The algorithm preferably doubles the recalibration interval if the extent of the thermal drift is less than a recalibration-doubling threshold. The algorithm preferably halves the recalibration interval if the extent of the thermal drift exceeds a recalibration-halving threshold. The algorithm preferably does not change the recalibration interval if the extent of the thermal drift is greater than the doubling threshold but less than the halving threshold. The algorithm preferably controls the micro-imaging system to backtrack to a location on the surface of interest to recapture the images taken since a last recalibration if the extent of the thermal drift exceeds a recapture threshold, and the algorithm preferably abandons image capture if the extent of the thermal drift exceeds an abort threshold. In addition, if the recalibration interval is doubled, the algorithm preferably compares the doubled recalibration interval with a predetermined maximum recalibration interval and sets the recalibration interval to the predetermined maximum recalibration interval if the doubled recalibration interval exceeds the predetermined maximum recalibration interval. If the recalibration interval is halved, the algorithm preferably compares the halved recalibration interval with a predetermined minimum recalibration interval and sets the recalibration interval to the predetermined minimum recalibration interval, if the halved recalibration interval is less than the predetermined maximum recalibration interval.
The focusing algorithm preferably performs a coarse focus search by capturing a series of images at a plurality of coarse focus settings and selecting a best coarse focus setting based on a derived focus measure for each captured image until a peak coarse focus measure is located. The focusing algorithm then performs a fine focus search centered around the peak coarse focus setting by capturing a series of images at fine focus settings and selecting a best fine focus setting based on a derived focus measure for each captured image in the fine focus search.
The advantages include an automated image capture system that enables seamless assembly of tile images into photo-mosaics, and an improved throughput of tile image mosaics.